Riser termination

ABSTRACT

A top termination ( 70 , FIG.  2 ) lies at the top of a riser ( 22 A) that extends vertically from the sea floor into a moon pool of a floating unit, or vessel ( 12 ) that is drilling or servicing a well, wherein the riser transfers fluids such as used drilling fluids up to the vessel. The vessel can move vertically in heave as well as tilt without requiring the top termination to follow such motions. The top termination carries fluid to the vessel through jumper hoses ( 74 ) that bend to take up vessel vertical motion and tilt. The weight of the top termination is carried by the vessel through flexible lines ( 44 A) that extend from near the top termination up to the vessel.

CROSS-REFERENCE

Applicant claims priority from U.S. provisional patent application No. 61/246,916 filed Sep. 29, 2009.

BACKGROUND OF THE INVENTION

In order to facilitate the explanation of the invention, applicant uses the term “vessel” to describe any type of structure that floats at the surface of a body of water. Applicant uses the term “sea” to refer to any salt or fresh water body of water. Applicant uses the term “moon pool” to describe a space at the height of a vessel hull where work can be done, which is usually a space surrounded by the hull but which can lie outside the hull.

During the drilling or servicing of a subsea well into a sea floor, a drill string or other string (group of series connected devices), extends down from a vessel or other structure that floats at the sea surface, and through a riser that extends vertically to the sea floor. The string may be hollow to carry fluid such as drilling mud down to a drill bit or other tool at the bottom of the string. Used fluids such as drilling mud then passes up through an annular space between the outside of the string and the inside of the riser, to a top termination at the top of the riser, that connects to the vessel where the fluids are processed for reuse.

While the riser extends at a fixed height above the sea floor and is stationary, the vessel translates and rotates, as by tilting in roll and pitch and moving vertically in heave as waves pass by. Tilting can be accommodated by flexible joints, but heave movement has adverse consequences. The top termination preferably lies in a vessel moon pool, with a work floor at the top of the moon pool. In the prior art, heave was accommodated by the vessel being connected to the top termination through a telescoping joint. The telescoping joint included a long inner pipe hanging from the vessel, that telescoped into a tall outer pipe extending upward from the top termination. The telescoping joint typically accommodates vertical heave of up to about 50 feet (15 meters) or more, and could be of a multi-barrel type. A taller space is required to accommodate the telescoping joint because one pipe must lie within the other pipe along a considerable length to produce a long stroke telescoping joint with limited function. The long length of telescoping pipes resulted in the need to leave a tall vertical space in the vessel (or outside it) between the water level and the underside of the work floor or the like, resulting in the need to locate the work floor (from which a string such as a drill string or other parts are launched) high above the vessel main deck. Also, it has been difficult to provide reliable seals along the telescoping joint, resulting in multiple seal packages which require frequent maintenance or replacement.

SUMMARY OF THE INVENTION

In accordance with one embodiment of the invention, an offshore hydrocarbon or other development system is provided that connects a top termination that lies at the top of a riser, to a floating unit such as a vessel, in a connection that avoids the need for a telescoping joint to accommodate motions of the vessel, especially heave. The system includes a plurality of flexible jumper hoses that each extends in a U-shaped or other curve between the top termination and a tank on the vessel in which fluid is collected. When the vessel translates and pivots, the jumper hose flexes to accommodate such motions.

The vessel supports the weight of the riser string through flexible tension lines or other means of tensioning that extend from the vessel to the top terminal. Except for the flexible lines and the jumper hoses, the top termination is free of connections to the vessel. This allows the vessel to undergo motion (e.g. roll and pitch) without comparable motions of the top termination, to minimize the need for flexible joints to connect the top termination to the vessel.

The novel features of the invention are set forth with particularity in the appended claims. The invention will be best understood from the following description when read in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevation view of a prior art offshore hydrocarbon or other development system which has a telescoping joint.

FIG. 1A is a sectional view of a drill string penetrating the sea floor.

FIG. 2 is a side elevation view of an upper portion of an offshore development system of the present invention.

FIG. 3 is a side elevation view of the top termination of the riser and adjacent portion of the system of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows an offshore development system 10 of the prior art, such as a hydrocarbon development system, which includes a floating unit, or vessel 12 that floats at the sea surface 14 and connects to a riser 20 that extends from near (within 20% of the sea height) or at the sea floor 22 up to the vessel. The system can be used in a construction phase (to drill holes in the sea bed) as well as in a testing and maintenance phase (well services, workover, intervention) and an abandonment phase. The system also can be used for the development of mineral deposits or entities requiring a fixed connection between a location near the sea floor and the vessel. A variety of different types of floating units can be used, all of which are sometimes referred to herein as a vessel. In FIG. 1, the system includes a derrick 24 or other means of hoisting a string, that supports a string within the riser, to drill or service a well in the sea floor. The figure shows a subsea stack 30 at the sea floor, that includes a disconnection section 32 comprising a well control and (dis)connection system, and a blowout prevention section comprising a well control section and a connection/disconnection section 34.

FIG. 1A shows an assembly 112 that lies in the riser 20 and that is drilling a hole in the sea floor 22. Drilling fluid is pumped down through the hollow center 36 of the string. Used fluid then flows upward through an annular region 38 for processing and reuse.

The riser 20 extends primarily vertically from the sea floor up to a coupling 40 that includes a riser tensioning ring that lies in a vessel moonpool 42 (in or beside a side or end of the vessel). The moonpool provides access to the riser bore. A moonpool in the vessel hull comprises an opening (usually square or otherwise rectangular) extending through the height of the vessel hull, that provides room for working around systems such as the upper part of the riser, and that extends up through the height of the bottom of the hull. A plurality of wires 44 or other means of tensioning extend up from a tensioning ring 46 to an upper portion of the vessel, to support the riser. The vessel undergoes motion (e.g. pitch and roll), but the riser is isolated from them by flexible couplings 40, 52 at the top of the telescopic joint and at the bottom of the riser.

The vessel 12 undergoes heave motion that may move the vessel vertically up or down by up to 15 meters or more. In the prior art system of FIG. 1, such vertical movement of the vessel relative to the riser is accommodated by a TJ (telescopic joint) 60. In the TJ 60, a long vertical inner pipe hangs from the vessel and extends into a long vertical outer pipe that lies in the tensioning ring. For a maximum anticipated vertical movement of the vessel, the TJ must be about 20% longer than the vertical movement. This is necessary so that adequate lengths of interfitting pipes remain even when the vessel is at its highest or lowest. The long lengths required result in the work floor 64 where the supported part of the derrick or other hoisting means lies and where string sections, tools, etc. are lowered into or retrieved from the riser, lying high above the bottom 66 of the moon pool. Also, seals that seal pipes that slide one within the other, are more difficult to access and maintain than devices that do not require seals to accommodate movement.

FIG. 2 shows a portion of a vessel 12A of the present invention, and the upper portion of a riser 22A of the present invention. The vessel 12A is similar to that of FIG. 1. The top of the riser has a top termination 70 that lies on the rest of a riser and at or near the riser vertical axis 73. The vessel has at least one fluid collecting tank 72 that receives used fluids (e.g. drilling mud) from the riser through a plurality of jumper hoses 74 that connect to the top termination 70. The fluid passes to a fluid treatment module 41 that processes the fluid for reuse. Each jumper hose 74 has one rigid pipe section 76 with an inner end that connects to the upper end of the riser and with an outer end 82 that extends downward. Each jumper hose has an opposite rigid pipe section 84 with an inner end that connects to the collecting tank 72 and an outer end 85 that extends downward. Each jumper hose also has a flexible hose portion 90 with ends 86 that extend downward from the rigid pipe sections, and has a middle 87 that extends primarily in a half circle, or 180° loop. When the vessel and its collecting tanks move up and down during vessel heave, the flexible hose portion 90 bends to maintain the fluid connection.

The top termination 70 of the riser is supported by flexible tension lines 44A that extend from a tensioning ring 46A around sheaves to a tensioner 92. The tension lines support the riser to locate it within the moonpool 66A, and the tensioner elongates and shortens the tension lines when the vessel moves up and down. Other means of tensioning also can be used to support the upper end of the riser. Applicant notes that the riser 20A terminates at the top termination 70 and is not connected to the vessel except through the flexible jumper hoses 74 and the flexible tension lines 44A. As a result, vessel motions can be accommodated without the riser undergoing corresponding motions, because the tension lines 44 can bend, and without requiring the flexible pivot joint at 40 of FIG. 1. Other means for tensioning the riser upper end preferably provide tension while allowing movement of the riser upper end. Applicant notes that similar jumper hoses and flexible tensioning means can be used when the moonpool lies at or beyond the bow, stern, or side of the vessel instead of in a pool surrounded by the hull.

FIG. 3 shows additional details of the system of FIG. 2, showing that the top termination 70 can support blowout preventers, including double ram preventers 100, single ram preventers 102 and annular preventers 104. A rotating head blowout preventer 106 lies near the top of the termination. A dedicated flow divider 108 divides fluid flow between the two jumper hoses, and is used to change vertical flow to horizontal flow. A collector funnel 110 lies at the very top of the termination, and serves to guide components such as a drill string 112 into the riser 20A.

Thus, the invention provides a hydrocarbon, mineral, or other development system which includes a riser that extends up from the sea floor to a top termination that lies in a space (e.g. moonpool) in a vessel hull or beside the hull, which allows the vessel to move relative to the riser and relative to a termination at the top of the riser, in a system of high reliability. The top termination is connected to one or more fluid collecting tanks on the vessel, through one or more jumper hoses that include flexible hose sections that maintain fluid connections of the riser to the vessel despite motions of the vessel. Such fluid connections are maintained without the need for a telescoping sliding connection. The top termination is connected to the vessel only through the jumper hoses or other fluid connection means and through flexible tension lines or other tensioning means, so the vessel can execute motions, especially heave and tilt (roll and pitch) without requiring the riser to execute corresponding motions.

Although particular embodiments of the invention have been described and illustrated herein, it is recognized that modifications and variations may readily occur to those skilled in the art, and consequently, it is intended that the claims be interpreted to cover such modifications and equivalents. 

1. An offshore development system that includes a floating unit that floats at the sea surface, and a riser string that extends from the sea floor to the floating unit, said riser string having an upper portion with a primarily vertical axis, comprising: a top termination (70) lying at the height of the floating unit, said top termination lying at an upper end of the riser string; at least one fluid collector tank (72) mounted on said floating unit and spaced from said axis; at least one jumper hose (74) that has a first end connected to said riser at said top termination and a second end (86) connected to said fluid collector tank, said jumper hose having a loop (90) coupled to said first and second ends of said hose with said loop being flexible so each of said ends is moveable vertically relatively to the other end by bending of the loop; termination biasing means (44A) for applying an upward force to said top termination to support said top termination at the height of said floating unit while allowing said floating unit to move vertically relative to said top termination.
 2. A system described in claim 1 wherein: said means for applying an upward force includes at least one flexible tension line (44A) that extends at least partially upward from said top termination to said floating unit, said flexible tension line maintaining tension on said top termination when the floating unit undergoes motions; said jumper hose is flexible along said loop, and said jumper hose bends when the floating unit undergoes motions; said top termination is free of connection to said floating unit except through said at least one flexible tension line and said at least one jumper hose, to allow said floating unit to undergo motions without comparable movement of said top termination.
 3. A system described in claim 1 wherein: said at least one jumper hose includes a first rigid pipe section (76) with an inner end connected to an upper end of said riser, a second rigid pipe section (84) connected to said at least one collector tank (72), and a flexible hose section (90) that connects to said first and second rigid pipe sections; said flexible hose section having opposite ends that each extends downward from a corresponding one of said rigid pipe sections and said flexible hose section having a middle portion that extends in primarily a half circle.
 4. A system described in claim 1, wherein: said top termination includes a funnel (110) with a wide top end and with a narrower lower end that lies in line with said riser string.
 5. A method for coupling the top of a riser through which a string extends, so fluid passes up through a riser annular area around the string and flows to a collecting tank (72) on a floating unit, wherein the top of the riser is isolated from motion of the vessel, comprising: coupling a top termination (70) that lies at the top of said riser, to said collecting tank, through at least one flexible hose (90), and supporting said top termination on said floating unit through flexible tension lines (44A) to allow the floating unit to tilt relative to said top termination.
 6. The method described in claim 5 wherein: said step of coupling through at least one flexible hose includes extending said flexible hose in a loop (90) with opposite upper ends (86) and connecting said upper ends one to said top termination and one to said collecting tank. 